1. Field of the Invention
This invention relates to a field emission element.
2. Description of the Related Art
When the electric field at the surface of a metal or semiconductor is as large as 10.sup.9 V/m, electrons pass through the potential barrier because of the tunnel effect, thus entering an evacuated space at room temperatures. This phenomenon is called field emission. The cathode which emits electrons utilizing that principle is referred to as a field emission cathode (hereinafter referred to as FEC).
Recently, flat emission cathode FECs with micron structures have been able to be manufactured fully using semiconductor machining technology. Since elements each which has a great number of FECs acting as emitters formed on a substrate irradiate the fluorescent substance surface with electrons, they are used as electron emission sources for field emission displays (hereinafter merely referred to as FEDs), electron optical systems for lithography, or the like.
FIG. 7 is a perspective view schematically illustrating the basic structure of a Spindt field emission display. The field emission display includes a cathode substrate 1, cathode electrodes 2, gate electrodes 4, an insulating layer 8, openings 31, an anode substrate 32, and anode electrodes 33. The symbol A represents an anode lead-out conductor; C1 to Cn represent cathode lead-out conductors; and G1 to Gm represent gate lead-out conductors.
Stripe-shaped cathode electrodes 2 are arranged on the cathode substrate 1. The insulating layer 8 is formed to completely cover the cathode electrodes 8. Gate electrodes 4 are arranged in a stripe form on the insulating layer 8 in the direction perpendicular to the cathode electrodes 8. The so-called Spindt field emission cathode is used for the above-mentioned FEC. Plural openings 31 are formed at each of intersections where the cathode electrodes 2 cross the gate electrodes 4 so as to penetrate the gate electrode 4 and the insulating layer 8 underlying the same. The cone electrode 5 (to be described later with reference to FIG. 9) is formed on the cathode electrode 2 in each opening. The cone electrode 5 acts as an emitter electrode.
The anode electrode 33 and a fluorescent substance layer (not shown) are formed on the lower surface of the anode substrate 32 such as a glass substrate. A positive voltage is applied on the anode electrode 33 via the anode lead-out conductor A. Image signals are respectively applied to the cathode electrodes 2 via the cathode lead-out conductors C1 to Cn. Drive signals are respectively supplied to the gate electrodes 4 via the gate lead-out electrodes G1 to Gm. In a display operation, the cone electrode disposed within each of the openings 31 emits electrons to glow the fluorescent substance coated on the anode electrode 33. In the case of the three primary color field emission display, stripe-shaped anode electrodes 33 corresponding to fluorescent substance luminous colors (not shown) are arranged in parallel to the cathode electrodes 2 and are connected to different anode lead-out conductors.
FIG. 8 is a plan view schematically illustrating the basic structure of a Spindt field emission display. Like numerals represent the same constituent elements as those shown in FIG. 7 and hence the duplicate description will be omitted. Numeral 6 represents a seal and 34 represents an insulating support.
Plural insulating supports 34 are set up on the insulating layer 8 (shown in FIG. 7) to sustain the gap between the cathode substrate 1 and the anode substrate 32 to a predetermined distance against the atmosphere pressure. The inside of the assembly is maintained at a high vacuum by placing the seal 6 such as a low melting point seal glass (e.g. fritted glass) and then thermally welding it.
In FIG. 8, the seal 6 is depicted to be somewhat to the inside from the fringe of the overlapped portion. However, in actual, the seal 6 is welded over the fringe or the area adjacent to the same. Cathode terminals C led out of the cathode electrodes 2 are arranged on the lower end portion of the cathode substrate 1. Similarly, gate terminals are arranged on the insulating layer 8 (shown in FIG. 7) overlaying the left end portion of the cathode substrate 1. An anode terminal A extending from the anode electrode 33 is formed on the upper end portion of the anode substrate 32.
FIG. 9 is a cross-sectional view illustrating a conventional field emission cathode and partially taken along one gate electrode 4. Referring to FIG. 9, like numerals represent the same constituent elements as those in FIG. 7. Numeral 3 represents a resistance layer; 5 represents a cone electrode; 6 represents a seal; and 41 represents a seal protective layer.
Cathode electrodes 2 of aluminum is formed on the cathode substrate 2 such as a glass. A resistance layer 3 of amorphous silicon (a-Si) is formed so as to cover each cathode electrode 2. An insulating layer 8 such as silicon dioxide (SiO.sub.2) film is formed on the resistance layers 3 and the area where the cathode electrodes 3 and resistance layers 3 stripe-shaped are not formed.
Gate electrodes 4 are stripe-shaped on the insulating layer 8 in the direction perpendicular to the cathode electrodes 2. Each of cone electrodes 5 is positioned within the opening formed through each gate electrode 4 and the insulating layer 8 and is formed on the cathode electrode 2 via the resistance layer 3. The cone electrode 5 is made of a metal such as molybdenum. The tip of the cone electrode confronts the anode electrode 33 though the opening. In the figure, only one cone electrode 5 is depicted in the width direction of the anode electrode 2. However, a large number of cone electrodes 5 are formed on the anode electrode 2.
Since the distance between the gate electrode 4 and the tip of the cone electrode 5 is of the order of submicrons, the cone electrode 5 can field-emit electrons by applying a small voltage of several volts between the gate electrode 4 and the cone electrode 5. Thus, an electron emission source is formed of the cathode electrode 2, the cone electrode 5, and the gate electrode 4. The resistance layer 3 restricts an excessive current flowing through the cathode electrode 2.
With no resistance layer 3, if a discharge or short circuit occurs between the gate electrode 4 and the tip of one cone electrode 5 due to a certain cause, an excessive current may flow between the gate electrode 4 and the cathode electrode 2, thus resulting in a breakage of both the lines. The resistance layer 3 prevent such excessive current. Moreover, if there is a cone electrode 5 which tends to easily emit electrons among a large number of cone electrodes 5, electrons intensively emitted from the cone electrode 5 may produce an abnormal bright spot on the screen. When a cone electrode 5 starts to emit excessive current, the resistance layer 3 produces a voltage drop thereacross, thus decreasing the voltage to be applied to the cone electrode 5. As a result, the electron emission is suppressed so that the cone electrode 5 can stably emit electrons.
The gate electrodes 4 need to bridge the sealed portions of the seal 6 to be led out. However, the seal protective layer 41 which covers the gate electrodes 4 at the sealed portion is made of silicon dioxide (SiO.sub.2). The seal protective layer 41 is sealed with the seal 6. Niobium (merely abbreviated as Nb) is used for the material of the gate electrodes 4. With no seal protective layer 41, the gate electrodes 4 of niobium is in contact with the fritted glass being a material of the seal 6. In this case, the fritted glass oxidizes the gate electrodes 4 at the electrode lead-out portions during the heating process for sealing, thus delaminating the gate electrode 4 from the insulating layer 8. Such delamination causes the seal 6 to intrude into the split portion and finally occurs a slow leakage phenomenon by which the vacuum degree of the envelope decreases gradually in a long period of time. Furthermore, the oxidation cause either an increased resistance of the gate electrode or a conduction failure of the gate electrode 4 due to a line breakage. For that reason, the seal protective layer 41 is disposed to prevent the gate electrode 4 to be in contact with the seal 6.
In the conventional field emission cathode, since the gate electrodes 4 care formed on the insulating layer 8 and the terminals for the cathode electrodes 2 are formed on the cathode substrate 1, so that the gate terminals and the cathode terminals are formed on difference layers respectively. This requires implementing different lead-out fabrication steps. Moreover, in order to isolate the gate terminals from the seal 6, it is necessary to carry out the steps of depositing the seal protective layer 4 and then patterning the same. Hence, the problem is that the conventional manner leads to increasing the number of steps and complicating the fabrication process.